Since the 1930's, camera manufacturers have had to provide a viewing device to give the photographer (still and motion picture) a means to view what the camera is seeing as the picture is being taken. When the manufacturers progressed and became more sophisticated in the manufacturing of their camera equipment, both cinema and still, especially the small, hand held 35 mm cameras, they added the ability to interchange lenses, thereby changing a given camera's effective focal length over a range extending from telephoto to wide angle. When they added this feature, they had to give the photographer a means to view what each lens was seeing and registering on the film.
To do this, some manufacturers developed rather sophisticated zoom lens systems that acted as the viewer for the camera. Other manufacturers developed viewing devices that were associated with their camera that would give the maximum angle of view for the widest angle lens they offered for an individual camera, and they put small scribed lines on the viewing aperture indicating the respective views encompassed by their other lenses for that camera. In the mid 1950's, manufacturers began merchandising single lens reflex viewing systems wherein the actual image the eye is seeing is being projected to the eye through the use of a system of prisms and/or mirrors, and the image is what the lens itself is actually viewing.
Viewing devices, however, remain in common use, particularly for cameras of the non-reflex type. When a lens is changed, one dials or otherwise sets a given viewing device to cause same to see what the viewing lens sees. Usually, the device has no intermediate markings or intermediate positions between those showing focal lengths of the lenses available for a particular camera only, click stops being customary. Furthermore, many of these devices utilize from four to seven lens elements, and apparently all employ complex lens movement and/or focusing means. There was no viewing device which would allow one to continuously alter focal length or to hold any desired particular focal length. So far as is known, there has not previously been available a simple, three lens instrument which would enable a photographer, or someone advising a photographer, to see and determine for himself what portion of a given general scene is to be photographed with a given lens, or what lens to use to photograph a desired portion of such a given general scene.
An object in photography is to end up with a picture composed of desired subject matter. In photographing a particular scene, this composition is obtained by selecting a lens having a focal length such that the desired field of view for such composition is obtained. While it is possible in a single lens reflex camera to have a built-in view finder that allows through the lens viewing of the exact scene which will be photographed, it will be understood and appreciated that, where the photographer has a large number of lenses from which to choose, the selection of an individual lens will be extremely time consuming, inconvenient, and, commonly, inaccurate. The most reliable procedure involves attaching a particular lens to the camera body, focusing on the desired scene, detaching that lens, and replacing that with another, etc., and then finally comparing and selecting from among the various views the lens providing the best view that he will have observed. There is a very real need among photographers for very simple, inexpensive, easily carried instrument which will allow them to preselect a particular lens focal length for a given subject using their camera, such that the picture to be taken may be easily and reliably composed or "framed."